Descent of ManIn this lecture, beginners can familiarize themselves with basic information and terms used to describe the evolution of humanity beginning with the origin of primates through the comings and goings of Genus Homo.

We began talking as early as 2.5m years ago, writes Stephen Oppenheimer. Is that what drove the growth of our brains?

When did we start talking to each other and how long did it take us to become so good at it? In the absence of palaeo-cassette recorders or a time machine the problem might seem insoluble, but analysis of recent evidence suggests we may have started talking as early as 2.5m years ago.

There is a polar divide on the issues of dating and linking thought, language and material culture. One view of language development, held by linguists such as Noam Chomsky and anthropologists such as Richard Klein, is that language, specifically the spoken word, appeared suddenly among modern humans between 35,000 and 50,000 years ago and that the ability to speak words and use syntax was recently genetically hard-wired into our brains in a kind of language organ.

This view of language is associated with the old idea that logical thought is dependent on words, a concept originating with Plato and much in vogue in the 19th century: animals do not speak because they do not think. The advances in communication and abstract thought demonstrated by chimps and bonobos such as the famous Kanzi put this theory in doubt.

The notion of a great leap forward in the quality of human thinking is further reflected in a common interpretation of the flowering of Upper Palaeolithic art in Europe. European cave paintings in Lascaux and Chauvet in France and carved figurines that have been dated to over 30,000 years ago are seen, according to this perspective, as the first stirrings of symbolic and abstract thought and also of language.

The problem with using art as prehistoric evidence for the first human that could speak is that, quite apart from its validity, the further back one looks the more chance the evidence for art itself would have perished.

An alternative to the Chomskian theory, is that language developed as a series of inventions. This was first suggested by the 18th-century philosopher Etienne Bonnot de Condillac. He argued that spoken language had developed out of gesture language (langage d'action) and that both were inventions arising initially from the simple association between action and object. The Condillac view, with some development, can be traced to the present day with the recent work of New Zealand psychologist Michael Corballis and others. The theory sees gesture language as arising originally among apes as sounds accompanying gestures, with these sounds gradually becoming coded into "words" as the new skill drove its own evolution. Subsequently, coded words developed into deliberate, complex communication. Evolutionary pressures promoted the development of an anatomy geared to speech - the larynx, vocal muscles and a specific part of the brain immediately next to that responsible for gestures.

This view, that spoken language was ultimately a cultural invention like tool-making, which then drove the biological evolution of the brain and vocal apparatus, seems obvious when you think of the development of different languages.

The unique features of a language such as French clearly do not result from any biological aspect of being French but are the cultural possessions of the French-speaking community. Each language evolves from one generation to the next, constantly adapting itself to cope with the learning biases of each new set of young, immature minds.

Several skull and spinal modifications relating to speech production (arched base of skull and enlargement of the channel for nerves to the tongue in early human fossils, a lopsided brain and changes in relative proportions of the brain) have all been used to shift speech way back to early humans 2.5m years ago or even earlier.

Anthropologists and fossil experts who accept that speech started early, still tend to think of language evolution as a gradual 2m year process with our own modern human species (Homo sapiens) way out at the top and our older human ancestors cast as mumbling, hooting parodies of ourselves. A major reason for this is the perception that brain growth among humans was gradual over the same 2.5m year period. Several recent changes in the fossil evidence bring this into doubt.

The first of these is a redating of soil layers from the famous Olduvai Gorge in east Africa where many key fossil remains have been found. A number of big-brained human species appear to be much older than previously thought, with several specimens dating over a million years old. When brain sizes for all available skulls are plotted against time, using the revised dates, the result is startling: the bulk of increase in brain size was over by around 1.2m years ago with some African human species having brain volumes easily within the modern human range by that time. Those in our own African ancestry stopped growing their brains perhaps 200,000 years ago and even started shrinking them over the past 150,000 years - the period of our own species' time on Earth.

So we have the paradox that over the period when our brain was growing most rapidly, our material cultural development, as measured by stone tools, advanced only marginally; then, over a million years later, when the culture of anatomically modern humans finally started to accelerate, artistically and technologically, our brains were actually getting smaller.

The additional piece of evidence that makes this paradox all the more significant is that brain size did not just leap between human species in a direct line of ascent towards ourselves. Over the period from 2.5 to 1.5m years ago, it turns out brains were growing more rapidly than at any time since, within all the different human species and also in Paranthropus species. The logical conclusion is that there must have been a unique new behaviour driving brain growth, shared between all species of humans and Paranthropus, with its origin, presumably, in their immediate shared walking ape ancestor.

So, what was driving rapid brain growth right at the beginning 2.5m years ago? The answer may have been staring us in the face. Namely, that not only were early humans and Paranthropus communicating but their ancestor, a walking ape, had started the trend in this very useful skill. Around 2.5m years ago the weather took a decided turn for the worse, becoming more variable and colder and dryer. The search for food became more taxing, and there would have been a real need to communicate more effectively and cope with the worsening environment in a cooperative way.

Speech, a complex system of oral communication, is the only inherited primate skill that would self-evidently benefit from a larger computer than that of a chimp. The near maximum in brain size achieved by 1.2m years ago indicates that those early ancestors could already have been talking perfectly well. It was all over bar the shouting. Our new Rolls Royce brain, developed to manipulate and organise complex symbolic aspects of speech internally, could now be turned to a variety of other tasks.

So what happened in the million gap years after that? Why did we take so long to get to the moon? Cultural evolution aided by communication and teaching is a cumulative interactive process. If each new generation invented just one new skill or idea and passed it on with the rest to their children and cousins, you could predict exactly the same curve of cultural advance as we see from the archaeological and historical record - first very slow, then faster and faster.